Apparatus for facilitating the descent of cable-suspended well tools through deviated well bores

ABSTRACT

As one embodiment of the new and improved apparatus disclosed herein, a well tool having force-responsive signalling means thereon is dependently coupled to an electrical logging cable that is uniquely constructed to withstand limited axial compressive loading without undue lateral bending. In this manner, by selectively moving the suspension cable downwardly from the surface, corresponding axial forces are imposed on the well tool for assisting its continued descent through substantially-deviated well bore intervals. The motion-responsive signalling means are responsive, therefore, to the application of such forces to the tool for indicating that these axial compressive forces are being transmitted through the cable to the tool.

United States Patent Stanley T. Dueker;

[72] Inventors John E. Key, Jr., Houston, Tex.

[21] Appl. No. 821,254

[22] Filed May 2, 1969 [45] Patented Mar. 2, I971 [73] Assignee Schlumberger Technology Corporation New York, N.Y.

[54] APPARATUS FOR FACILITATING THE DESCENT OF CABLE-SUSPENDED WELL TOOLS THROUGH DEVIATED WELL BORES 4 Claims, 4 Drawing Figs.

[52] U.S. Cl. 200/52,

200/18, ZOO/61.42, ZOO/61.45

[51] lnt.Cl H01h 35/14,

HOlh 36/00 [50] Field of Search 200/19 (M), 61.45 (M), 67 (F), 81.9 (M), 83.6, 84.3,147 (A), 52 (TMS), 6142,18, 52; 335/205, 206, 207

[56] References Cited UNITED STATES PATENTS 2,719,485 10/1955 Bendar 335/205 2,869,475 1/1959 Bobo 200/84(.3)X

3,105,551 10/1963 Ehlert... 200/84(.3)X

3,198,904 8/1965 Ehlert... 335/205 3,261,934 7/1966 Kisling 200/6 1 .42X

3,303,304 2/l 967 Campbell, Jr. et al. 200/6l.42X

3,419,695 12/1968 Dinkelkamp et al ZOO/84(3) 3,421,124 l/l969 Kidd 200/6L4IX 3,437,771 4/1969 Nusbaum ZOO/84(3) OTHER REFERENCES IBM Technical Disclosure Bulletin, Bolan et al. Vol. 8, No. 7, Dec. 1965 Copy Available 1a Class 200, Sub. 83.6

Primary Examiner-Robert K. Schaefer Assistant Examiner--J. R. Scott Attorneys-Ernest R. Archambeau, Jr., William J. Beard,

David L. Moseley, Edward M. Roney, William R. Sherman and Stewart F. Moore ABSTRACT: As one embodiment of the new and improved apparatus disclosed herein, a well tool having force-responsive signalling means thereon is dependently coupled to an electrical logging cable that is uniquely constructed to withstand limited axial compressive loading without undue lateral bending. In this manner, by selectively moving the suspension cable downwardly from the surface, corresponding axial forces are imposed on the well tool for assisting its continued descent through substantially-deviated well bore intervals. The motion-responsive signalling means are responsive, therefore, to the application of such forces to the tool for indicating that these axial compressive forces are being transmitted through the cable to the tool.

PATENTEDHAR 21s?! Stanley T. Dueker 2 John E. Key, Jr.

INVENTORS ATTORNEY APPARATUS FOR FACILITATING THE DESCENT OF CABLE-SUSPENDED WELL TOOLS THROUGH 'DEVIATED WELL BORES In some oil fields, it is not at all uncommon to find well bores having highly-slanted intervals therein that may approach angles deviating from the vertical as much as 60 to 80". It is, of course, all but impossible to simply lower a cablesuspended well tool to the bottom of such highly-deviated well bores. It will be appreciated, therefore, that unless suitable techniques are devised for moving cable-suspended well tools through such highly-deviated holes, they cannot be logged or perforated in the usual manner.

Accordingly, it is an object of the present invention to provide new and improved apparatus for facilitating the descent of cable-suspended well tools through highly-deviated well bores.

This and other objects of the present invention are accomplished by coupling a well tool that is to be lowered into a well bore having one or more deviated intervals to a relatively-stiff suspension cable that is spooled on a typical powered winch. So long as the well too] continues to descent into the well bore, the cable is allowed to unreel from the winch at a desired speed. Once, however, the descent of the tool is slowed or checked as, for example, when the tool moves into a highlydeviated interval of the well bore, the cable is moved downwardly to reduce tensile forces therein and develop corresponding compressive forces in the lower portion thereof which are imposed on the tool. Means are provided for indicating at the surface whether this axial thrust is being applied to the tool. In this manner, since the cable has sufficient lateral rigidity that it cannot be doubled back into itself, these downwardly-acting compressive forces will be transmitted to the tool for promoting its further movement. If the tool is hopelessly halted, the cable will, however, ultimately become coiled into frictional engagement along the walls of the well bore. Thus, the signalling means of the present invention will indicate whether axial loads being applied to the cable at the surface are effective for moving the tool.

The novel features of the present invention are set forth with particularly in the appended claims. The invention, together with further objects and advantages thereof, may be best understood by way of the following description of exemplary apparatus employing the principles of the invention as illustrated in the accompanying drawings, in which:

FIG. 1A depicts a typical arrangement of the surface equipment employed for imposing downward forces to a well tool suspended from a logging cable in a well bore;

FIG. 1B shows a well tool as it may appear while being pushed through a deviated borehole during the practice of the present invention;

FIG. 2 is a cross-sectional view of the upper portion of an exemplary well tool including force-responsive signalling means arranged in accordance with the principles of the present invention; and

FIG. 3 is an isometric view, partially in cross section, of a unique suspension cable particularly adapted for use with the present invention.

Turning now to FIG. 1B, a well tool such as an electrical logging instrument or other well-completion tool is depicted as it is being moved through a highly-deviated interval of a well bore which, in this instance, is illustrated as being an uncased-borehole 11. Although the specific nature of the well tool It) is of no consequence to the present invention, it is preferred that the tool include some means, as at 12, for providing signals at the surface that are at least indicative of whether or not the tool is moving through the borehole 11. Thus, in one manner of providing such signals, where the tool 10 is a typical logging instrument, the signalling means 12 may include a typical radioactivity detector or other measuring device which responds to some characteristic of earth formations. On the other hand, if the tool 10 is a perforator or other completion tool for use in a cased well bore, the measuring device 12 could also be a typical casing collar locator. The well tool 10 is dependently coupled to the lower end of a uniquely-constructed suspension cable 13 which, as will subsequently be explained, is appropriately arranged to withstand substantial axial compressive loads without being doubled back onto itself within the confines of the borehole 11.

As seen in FIG. 1A, the logging cable 13 is spooled in the usual fashion on a powered winch 14 and has previously been directed into the surface casing 15 that customarily lines the walls of the upper portion of the borehole 11. As is typical, the winch I4 is positioned in a convenient location adjacent to the surface casing 15 and the cable 13 is preferably directed into the casing means of upper and lower pulleys or sheaves l6 and 17 aligned with one another and the winch and respectively supported directly above the casing and to one side thereof. It will be appreciated, of course, that the winch 14 is operatively equipped with brakes and a driving mechanism (neither shown) by which the winch drum may be selectively driven in either rotative direction and at any suitable rotational speed. The upper sheave 16 is typically supported in a derrick (not shown) by a strain gauge 18 that is coupled to a suitable indicator 19 for measuring the tension forces on the electrical logging cable 13. Asis usual, a calibrated measuring wheel 20 that is frictionally driven by the running portion of the cable 13 is coupled to a totalizer 21 for measuring the length of the cable being reeled onto or off of the winch 14 and a tachometer 22 for indicating the speed of the cable. A suitable instrument 23 is electrically coupled by way of the armor logging cable 13 to the measuring device 12 in the well tool 10.

Turning now to FIG. 2, the upper portion of the well too] 10 is shown in cross section to illustrate a preferred embodiment of new and improved signalling means 24 in accordance with the present invention and adapted for indicating when an axial force is being effectively imposed through the suspension cable 13 to the well tool. As illustrated, the lower end of the suspension cable 13 is suitably secured within a cylindrical body 25 that is slidably received within the open upper end of a tubular housing 26 coupled to the upper end of the well tool 10. To limit the upward longitudinal travel of the cylindrical body 25 in relation to the housing 26, an elongated sleeve 27 having an inwardly-directed shoulder 28 is secured within the housing to position the shoulder above the upper face 29 of an enlarged-diameter head 30 arranged on the lower end of the cylindrical body. Conversely, downward longitudinal travel of the cylindrical body 25 is limited by an enlarged-diameter shoulder 31 arranged around an intermediate portion of the cylindrical body to engage an upwardly-facing shoulder such as defined by the upper end 32 of the sleeve 27. To corotatively secure the cylindrical body 25 in relation to the housing 26, means such as a longitudinal spline-and-groove arrangement, as at 33, are provided on the enlarged head 30 and internal wall of the sleeve 27.

Accordingly, it will be appreciated that the cylindrical body 25 is free to travel axially in relation to the housing 26 within the limits provided between the opposed upper shoulders 31 and 32 and the opposed lower shoulders 28 and 29. For reasons that will subsequently become apparent, the cylindrical body 25 is biased upwardly in relation to the housing 26 by means such as a compression spring 34 coaxially disposed within the tubular housing and yieldably restrained between the shoulder 31 and a ring 35 mounted around the elongated sleeve 27.

As shown in FIG. 2, the cylindrical body 25 is not fluidly sealed in relation to the housing 26 so that the tubular housing will be filled with the borehole fluids. To prevent the borehole fluids from electrically shorting the various cable conductors, as at 36, the conductors are appropriately sealed within the cylindrical body 25 and connected, as at 37, to conductors, as at 38, leading to the interior of the well tool 10 and brought through typical conductor seals (not shown) sealingly mounted in a transverse partition 39 across the lower end of the tubular housing 26. Accordingly, inasmuch as the hydrostatic pressure of the borehole fluids will be acting on both sides of the cylindrical body 25, there will be no unbalanced pressure forces affecting the relative longitudinal position of the cylindrical body in relation to the tool housing 26, Thus, imposition of a downwardly-directed force through the cable 13 to the cylindrical body 25 will be effective to move the cylindrical body downwardly in relation to the housing 26 against the spring force of the compression spring 34. Similarly, in the usual situations, the weight of the well too] will be transmitted to the logging cable 13 by means of the opposed shoulders 28 and 29. Thus, it will be appreciated that so long as the well tool 10 is dependently suspended from the logging cable 13, the shoulders 28 and 29 will be abutted; and a downwardly-acting axial force at least as great as the potential spring force provided by the compression spring 34 will be required to shift the cylindrical body downwardly in relation to the tool housing 26 so as to bring the shoulders 31 and 32 into abutment.

Accordingly, it will be appreciated thatwhen the well tool 10 is suspended, the cylindrical body 25 will be at its depicted upper position in relation to the tool housing 26 so that the full weight of the tool is supported by the shoulder 28 and 29. Similarly, as the tool 10 is moving through a deviated well bore, such as the borehole l1, upwardly-directed forces on the housing 26 tending to slow or halt further progress of the well tool 10 will be countered by the longitudinal or axial component of the weight of the well too]. It will be seen, therefore, that when the cylindrical body 25 is elevated in relation to the tool housing 26, the summation of the downwardly-acting forces on the well tool 10 (such as the longitudinal component of the tool weight) is greater than the downwardly-acting forces, if any, on the cylindrical body. Conversely, when the downwardly-acting forces on the cylindrical body 25 are equal or greater than the upwardly-directed forces on the tool housing 26 (such as frictional drag) tending to slow or 'halt the well 10 as well as the spring force of the spring 34, the cylindrical body will be shifted downwardly to bring the shoulder 31 to abutment with the shoulder 32. Thereafter, so long as these downwardly-acting forces .on the cylindrical body 25 predominate, these downwardly-acting forces on the cylindrical body 25 predominate, these downwardly-acting forces will be effective for pushing the well tool 10.

Accordingly, in the preferred arrangement of the signalling means 24 of the invention for indicating the position of the cylindrical body 25 in relation to the tool-housing 26, a proximity switch, such as so-called reed switch 40 adapted for remote magnetic actuation, is encapsulated in a suitable pressure-resistant case and secured, as by spring clips 41, within the elongated sleeve 27. To actuate the encapsulated proximity-sensing switch 40, a magnet 42 is encapsulated in a suitable case 43 and dependently secured below the cylindrical body 25 for movement thereby into and out of the proximity of the switch. in this preferred embodiment, the switch 40 is normally open and the magnet 42 is longitudinally positioned in relation thereto for actuating the switch upon movement of the cylindrical body 25 to its lower position to shift the magnet case 43 to the position shown at the dashed lines at 44.

To provide means for adjusting the relative longitudinal positions of the magnet 42 and switch40, a depending rod 45 supporting the magnet case 43 is preferably threaded so that the magnet case can be screwed upwardly or downwardly along the rod. One lead 46 from the switch 40 is connected to a selected conductor 36a in the suspension cable 13 and another switch lead 47 is electrically connected to another cable conductor or to the tool housing 26 as at 48. Since the armor of the logging cable 13 is electrically connected to the housing 26, the cable armor will serve as a return path. In this manner, so long as the switch 40 is open, the cable conductor 364 will not be connected to any other cable conductors; and upon movement of the magnet 42 to its lower position 44, the switch will close to connect the cable conductor 36a to the tool housing 26. Thus, by connecting a suitable electrical instrument such as, for example, an ohmmeter 49 between the cable armor and the conductor 36a at the surface, a surface indication will be provided representative of the longitudinal position of the cylindrical body 25 in relation to the tool housing 26.

Turning now to FIG. 3, an isometric view is shown of a preferred embodiment of the suspension cable 13 to illustrate its unique construction. As fully explained in a copending application, Ser. No. 82l,l86 filed May 2, 1969, the logging cable 13 has a central axial core 50 comprised of a plurality of insulated electrical conductors 36 that are symmetrically grouped and encased in a suitable sheath 5] which, for exam ple, may be either braided Nylon strands or spirally-wrapped insulating tape. To give the core 50 a generally-cylindrical form, the interstitial spaces between the electrical conductors 36 and the sheath 51 are filled with semiconductive materials such as cotton tillers 52. To reduce the effects of capacitive coupling between the several cable conductors 36, the outer surfaces of the insulating sheath 53 of each cable conductor 36 is coated with a thin, electrically-semiconductive film and the cotton fillers 52 and Nylon braid 51 (or spiraled tape) are impregnated with an electrically-semiconductive compound. Thus, in the finished cable core 50, each insulated conductor 36 is surrounded by a semiconductive film which is electrically connected to the external armor 54 of the logging cable 13.

To protect the central cable core 50, the external armor 54 is comprised of a unique arrangement of four concentric layers 55-58 of helically-wound armor strands wound in a particular manner around the central core. The two innermost layers 55 and 56 of the armor 54 are assembled onto the central core 50in the usual manner. That is to say, the innermost layer 55 of the armor 54 comprises a plurality of metallic strands wound in one so-called lay"direction about the core 50; and the second layer 56 of the armor similarly comprises a plurality of metallic strands wound with an oppositelydirected lay about the innermost armor layer. The number, size and pitch or lay angle of the first and second layers are chosen so that the innermost layer 55 substantially covers the central core 50 and the second layer 56 substantially covers the first layer. As is customary, these inner layers 55 and 56 of the armor 54 perform the,usual tension-bearing functions of the cable 13 as well as protect the central cable core 50 from damage. a

It will be appreciated, of course, that if the logging cable 13 had only the two inner layers 55 and 56 of armor strands, as is typical for such cables, the cable would have only minimal longitudinal stability for withstanding axial compressive loads. Accordingly, inasmuch as the electrical logging cable 13 must also be capable of transmitting axiallydirected compressive forces through its lower portion to the well tool 10, the two outer layers 57 and 58 of the armor 54 are uniquely arranged to laterally stiffen the cable to withstand such compression forces without materially reducing the ability of the cable to be spooled onto the winch 14 and operatively carried over the sheaves 16 and 17. More significantly, these two outer layers 57 and 58 are uniquely wound in oppositely-directed lays and respectively have a pitch angle in the order of about 35as compared to only about l8for the two. inner layers 55 and 56.

Accordingly, by virtue of the substantial pitch angles of the two outer layers 57 and 58, the logging cable 13 will be incapable of doubling back on itself within the diameter of a typically-sized well bore. Thus, when compressive loads are imposed on the lower portion of the suspension cable 13, these loads will initially be transmitted through the cable to the tool 10 and gradually begin bending the lowermost length of the cable into a long helix. However, instead of doubling back on itself as would otherwise be usually expected, the logging cable 13 will continue transmitting axial compressive loads to the tool 10 until it has been helically looped so as to expand the cable loops into engagement with the walls of the borehole l1. Ultimately, of course, the frictional engagement of the helical loops of the cable 13 with the walls of the borehole 11 will. be sufficient to tightly wedge the cable so that further compressive loads cannot be imposed therethrough so long as the loops are against the borehole wall.

with this unusual ability of the logging cable 13 in mind, it will, therefore, be appreciated that as the well tool 10 is progressively moved downwardly through the borehole ll, axial compressive loads can be imposed thereon by way of the cable. Thus, to use the signalling means 24 of the present invention, the well tool is lowered into the borehole 11 at, preferably, as high a speed as can be attained by free fall of the well tool through the borehole fluids. This will, of course, mean that for most situations, the winch 14 will be allowed to freewheel so that the weight of the well tool 10 and progressively-increasing weight of the cable 13 hanging in the borehole 11 will be effective for carrying the well tool to as great a depth as is possible. It will be recognized, of course, that so long as the well tool 10 is moving relatively freely, the switch 40 will remain open. Once the well tool 10 enters a relatively-deviated interval of the borehole 11 as shown in MG. 18, the tool will, of course, ultimately be halted and come to rest in a position such as that illustrated. If the tool 10 does halt, this will be apparent at the surface since the electrical signal provided by the logging device 12 included with the tool will now show an unvarying signal on the instrument 23 in contrast to the usual varying signals that normally occur as the tool is moving.

When the operator at the surface has ascertained that the tool 10 has in fact come to rest, in one manner of practicing the present invention, the winch 14 can be powered forwardly (as at 59) in such a manner as to unreel an additional length of the cable13. Inasmuch as the portion of the cable 13 running over the sheaves l6 and 17 and a substantial proportion of its length within the borehole 11 will be under extreme tension, powering of the winch 14 will in effect merely reduce this tension load in the major portion of the logging cable. At some point, however, very near to the tool 10, the tensile load on this lower portion of the cable 13 will be zero so that any further downward force on the cable will induce a compressive force in the portion of the cable therebelow. Thus, the lowermost portion of the cable 13 will instead have an axial compressive load imposed thereon which is, of course, transmitted downwardly to the slidable cylindricalbody 25 in the upper end of the tool housing 26.

If this compressive load is less than the opposing spring force provided by the compressing spring 34, a corresponding downwardly-directed force will be transmitted by of the spring to the tool housing 26 which, hopefully, will be sufficient to move the well tool 10 still further. If, on the other hand, this downwardly-acting compressive load on the logging cable 13 is greater than the opposing spring force provided by the spring 34, the cylindrical body 25 will move downwardly in relation to the too] housing 26; and, once the magnet 42 reaches the position 44 opposite the switch 40, the switch contacts will close and provide an indication at the surface on the ohmmeter 49 that suchhas occurred. This information will, therefore, advise the operator at the surface that a force of at least as great as the spring force of the spring 34 is being applied to the tool 10 for moving it further downwardly in relation to the borehole 11. If the well tool 10 does in fact begin to move further downwardly, the logging device 12 will again provide varying electrical signals indicative of the different formation materials being passed by the moving well tool. On the other hand, if the downward forces are not sufficient to move the well tool 10, the steady reading on the instrument 22 provided by the logging device 12 will advise the operator that the tool is not moving. Moreover, the ohmmeter 49 will provide an indication of whether these downward forces are actually being imposed on the tool 10. Then the winch 14 can be driven forwardly, as at 59, to insure that the cable 13 is hanging free which will be indicated by substantial flexing of the portion of the logging cable between the lower sheave 17 and the winch. Once the cable 13 has been significantly flexed in this running portion of the cable, the cable can be pushed downwardly by hand or some cable-pushing device at the surface to impose a maximum load on the tool 10. It is, of course, impossible to impose further loading through the cable onto the well tool 10 unless the cable is partially respooled onto the winch 14 and then the tool is again relowered in an attempt to gain sufficient momentum to pass the impediment.

it has been found, however, that in actual situations where a borehole, such as at 11, is substantially deviated, with ordinary suspension cables it has not been possible to get well tools as at 10 to intervals in the borehole beyond the uppermost highly-deviated portions thereof. On the other hand, by employing the new and improved force-responsive signalling apparatus of the present invention, a significant aid is provided in facilitating the descent of these well tools. Usually, by watching the recording instrument 22 connected to the logging device 12, an operator will be advised that the well tool 10 is beginning to slow. When this is noticed, the winch 14 may be powered forwardly, as at 59, to begin driving the suspension cable 13 to supplement gravitational and inertial forces moving the well tool 10 downwardly through the borehole 11. In this manner, by taking advantage of the motional inertia of the well tool 10, often the added impetus provided by positively driving the winch 14 to impose axial compressive forces on the moving well tool will carry the tool along substantially-deviated or nonvertical intervals and on into more-vertical intervals'therein.

Accordingly, it will be appreciated that the present invention has provided new and improved apparatus for facilitating the descent of well tools through highly-deviated well bores.

Thus, although changes and modifications may be made in the principles of the invention as set out in the claims, by providing the force-sensing mechanism in the well tool, this well tool may be moved by a relatively-stiff logging cable'through even highly-deviated boreholes to greater depths than has heretofore been attainable.

We claim: 7

1. Apparatus adapted for passage through well bores having deviated intervals therein and comprising:

a well tool having a body member with a longitudinal bore in the upper. end thereof; a supporting member slidably disposed in said longitudinal bore and adapted for-relative axial movement therein upon imposition of axial loads thereon; stop means arranged on said members for limiting axial movement of said supporting member to a span between upper and lower spaced positions within said longitudinal bore; a suspension cable including electrical conductor means, and an armor sheath operatively arranged around said conductor means and adapted for carrying axial compressive loads of a predetermined magnitude; means coupling said armor sheath to said supporting member and adapted for transmitting axial compressive loads from said armor sheath to said supporting member in its said upper position and responsive to axial compressive loads of a lesser magnitude imposed on said supporting member for allowing said supporting member to move toward its said lower position and impose such axial compressive loads on said body member; and switching means arranged on said well tool and including a switch mounted on one of said members and operatively coupled to said electrical conductor means, and switchactuating means mounted on the other of said members and operatively arranged for selectively operating said switch only when said supporting member moves to its said lower position to provide an electrical signal designating that axial compressive loads are being transmitted from said armor sheath to said body member.

2. The well tool of claim 1 wherein said retaining means include spring means normally urging said supporting member toward its said upper position in relation to said body member and adapted to yield only upon imposition of axial compressive forces onto said supporting member.

3. The well tool of claim 1 wherein said switch includes a plurality of electrical contacts, and means for selectively opening and closing said electrical contacts upon movement of a magnetic field into and out of the vicinityof said switch; and saidswitch-actuating means include a permanent magnet having a magnetic field of sufficient strength to actuate said switch only whenever said supporting member is in a selected one of its said positions.

and adapted to yield only upon imposition of axial compressive forces onto said supporting member. 

1. Apparatus adapted for passage through well bores having deviated intervals therein and comprising: a well tool having a body member with a longitudinal bore in the upper end thereof; a supporting member slidably disposed in said longitudinal bore and adapted for relative axial movement therein upon imposition of axial loads thereon; stop means arranged on said members for limiting axial movement of said supporting member to a span between upper and lower spaced positions within said longitudinal bore; a suspension cable including electrical conductor means, and an armor sheath operatively arranged around said conductor means and adapted for carrying axial compressive loads of a predetermined magnitude; means coupling said armor sheath to said supporting member and adapted for transmitting axial compressive loads from said armor sheath to said supporting member in its said upper position and responsive to axial compressive loads of a lesser magnitude imposed on said supporting member for allowing said supporting member to move toward its said lower position and impose such axial compressive loads on said body member; and switching means arranged on said well tool and including a switch mounted on one of said members and operatively coupled to said electrical conductor means, and switch-actuating means mounted on the other of said members and operatively arranged for selectively operating said switch only when said supporting member moves to its said lower position to provide an electrical signal designating that axial compressive loads are being transmitted from said armor sheath to said body member.
 2. The well tool of claim 1 wherein said retaining means include spring means normally urging said supporting member toward its said upper position in relation to said body member and adapted to yield only upon imposition of axial compressive forces onto said supporting member.
 3. The well tool of claim 1 wherein said switch includes a plurality of electrical contacts, and means for selectively opening and closing said electrical contacts upon movement of a magnetic field into and out of the vicinity of said switch; and said switch-actuating means include a permanent magnet having a magnetic field of sufficient strength to actuate said switch only whenever said supporting member is in a selected one of its said positions.
 4. The well tool of claim 3 wherein said retaining means include spring means normally urging said supporting member toward its said upper position in relation to said body member and adapted to yield only upon imposition of axIal compressive forces onto said supporting member. 